Challenges and opportunities in translating immobilized molecular catalysts for electrochemical CO2 reduction from aqueous-phase batch cells to gas-fed flow electrolyzers

Immobilized molecular catalysts (IMCs) with well-defined active sites and tunable coordination environments are promising candidates for catalyzing the electrochemical CO2 reduction reaction (CO2RR) with high activity and selectivity. With substantial progress in catalyst development, IMCs are being transitioned from batch cells (e.g., H-cells) where activity is limited by CO2 solubility in aqueous electrolyte, to flow electrolyzers equipped with gas diffusion electrodes (GDEs) that can achieve commercially relevant CO2RR current densities. This transition is challenged by the drastic differences in the microenvironment (e.g., local pH, CO2 concentration, GDE wetting) between batch cells and flow electrolyzers, and a poor understanding of the implications of these microenvironment changes on CO2RR performance of IMCs. In this perspective, we highlight recent studies that probe the IMCmicroenvironment interactions in GDE configurations, and we suggest strategies to understand better microenvironment effects using electrochemical measurements, in situ spectroelectrochemical measurements, and post-mortem catalyst characterization.